Geochemical Characteristics Of Eolian Deposits In Ganzi Area And Its Environmental Implications

Eolian deposits is closely related to atmospheric circulation and environment ofprovenance, therefore, it is an ideal geologic record to study formation and evolution ofatmospheric circulation as well as its mechanism in the history. In the river valley of thewestern Sichuan Plateau, there is scattered distribution of relatively older eolianloess-paleosol sequence. This eolian loess-paleosol sequence is a record of evolution of thesouthwest monsoon and plateau monsoon, and plays an important role in studies such asevolution of atmospheric circulation and climate change resulting from the uplift of theplateau. Geochemical methods, as one of the most important way in eolian deposits research,is commonly used in revealing chemical weathering process, evolution of ancient climate, andtracing of provenance. In this thesis, we measured and analyzed macroelements, traceelements, and Sm-Nd isotopic elements within loess and paleosol of different periods, whichare from Ganzi region in the eastern margin of the Qinghai-Tibet Plateau, aiming atinvestigation of chemical weathering process in eolian deposits of the Ganzi region, revealingancient climatic evolution laws of provenance and deposition area, and predicating thereasonable source.Initial results achieved are as follows:Based on percentage change of easily migrating microelements and change of chemicalweathering indices, we investigated chemical weathering process in eolian deposits of Ganzi,and distinguished chemical weathering indices of provenance and deposition area. Theconclusion is that eolian deposits in Ganzi region is in the primary weathering stage, mainlyexperiencing decomposition of albite, leaching of pedogenic carbonate, and valence change ofiron; and that the migration degrees of Ca*and Na indicate weathering strength of provenance,while leaching and deposition of pedogenic carbonate and valence change of iron correspondsto climate of deposition area.The change rule of weathering indices from provenance and deposition area showsdifference of ancient climatic evolution between provenance and deposition area since L3age:from L3age to L2age, the climatic evolution displayed opposite trends in provenance and deposition area-in provenance area climate became warm and wet and there was obviousdifference between winter monsoon and summer monsoon while in deposition area climatebecame cold and dry and there was no obvious difference between winter monsoon andsummer monsoon; during S1period, climate was cold and dry in provenance area whilerelatively warm and wet in deposition area; after S1period, provenance and deposition arearevealed similar climatic change and the difference between summer monsoon and wintermonsoon became apparently in both provenance and deposition area.We measured different samples from Ganzi and Lingtai, and based on contents andratioes of microelements with stable chemical properties and difference of Sm-Nd isotopicelements, we proved that provenance of eolian deposits from Ganzi is different with that ofloess from the Loess plateau. And from contents of tracing elements properties of samplesfrom Ganzi we primarily predicated that eolian deposits of Ganzi was mainly fromQinghai-Tibet Plateau itself: eolian deposits in Ganzi shows high contents of∑REE, Li, Cs,Rb, Bi, Zr and Hf, relatively higher contents of La/Nb, Hf/Nb, Th/Nb, and apparently lowcontents of Ba/Rb, Zr/Hf,Nd(0),147Sm/144Nd; and according to geological analysis ofsurrounding areas, we think that relatively higher contents of Zr, Hf and Bi are probablyrelated to large area developing acidic magmatite in Qinghai-Tibet Plateau, and that highcontents of Li, Cs, and Rb results from the unique high background value of Qinghai-TibetPlateau.According to contents and ratioes of stable elements and high amplitude fluctuation ofisotopic elements indices, we inferred that material source of eolian deposits in Ganzi is notstable, and the reason is that discrepant tectonic uplift movement of Qinghai-Tibet Plateauinduced property change of underlying surface as well as change of transportation force withdirection and strength, resulting in large fluctuation range of microelements TiO2/Al2O3,K2O/TiO2, rare earth elements∑REE, Eu, LREE/HREE, and tracing elements La/Nb, Hf/Nb,Th/Nb, La/Th.In summary, several achievements are obvious: Distinguish chemical weathering indicesof provenance and deposition area provides basic reference for the future paleoclimatic research in this area; Climatic evolution discrepancy between provenance and depositioncombined with source instability showed by geochemistry reveal the different impacts onclimate variability in different region caused by Qinghai-Tibet Plateau uplift. The research onthe determinated source area and its paleoclimatic evolution provide the possibility forunderstand the eolian produce reason and its transport models.